NETWORKS AND
APPLICATIONS
COMP SCI 3001
Faculty of Engineering, Computer and Mathematical Sciences
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Data Link Layer Introduction
Data Link Layer introduction
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
Data Link Layer introduction
• Examples:
– host-router, router-router, host-host
• Unit of data or PDU: frame (vs. IP packets)
data link protocol
phys. link adapter card
Ht M frame
application
Data Link Layer introduction
Where is the link layer implemented?
• in each and every host
• link layer implemented in “adaptor” (aka network interface card NIC) or on a chip
– Ethernet card, 802.11 card; Ethernet chipset
– implements link, physical layer
• attaches into host’s system buses
• combination of hardware, software, firmware
(e.g., PCI)
application
controller
transmission
network adapter card
Data Link Layer introduction
Data Link Layer introduction
Link layer services
• Encapsulate datagram into frame: add header and a trailer
• ‘Physical addresses’ used in frame headers to identify source and destination
– different from IP address!
2 Link access
– Implement channel access if shared medium
Data Link Layer introduction
Link layer services
3 Reliable delivery between two physically connected devices
• We learned how to do this already (remember TCP?)
• Seldom used on low bit error link (glass fiber, some twisted pair)
• Wireless links: high error rates
Q: why both link-level and end-end reliability?
Data Link Layer introduction
Link layer services (cont.)
Flow control
• Pacing between sender and receiver(s)
Data Link Layer introduction
Link layer services (cont.)
Flow control
• Pacing between sender and receiver(s)
Error detection
• Errors caused by signal attenuation and noise – every link will have errors
• Receiver detects presence of errors
– signals sender for retransmission or drops frame
Data Link Layer introduction
Link layer services (cont.)
Flow control
• Pacing between sender and receiver(s)
Error detection
• Errors caused by signal attenuation and noise
– every link will have errors
• Receiver detects presence of errors
– signals sender for retransmission or drops frame
Error correction
• Receiver identifies and corrects bit error(s) without resorting to re-
transmission
• Also called Forward Error Control
• Problem:morebitstotransmit!
Data Link Layer introduction
Link layer services
• Flow control
• Reliable delivery between two physically connected devices
Already looked at in TCP
• Error detection and Correction
• Link access
So we will consider
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